Washington state requires “a building owner or operator, or their agent, of a nonresidential building shall disclose the United States environmental protection agency’s energy star portfolio manager benchmarking data and ratings to a prospective buyer, lessee, or lender for the most recent continuously occupied twelve-month period. A building owner or operator, or their agent, who delivers United States environmental protection agency’s energy star portfolio manager benchmarking data and ratings to a prospective buyer, lessee, or lender is not required to provide additional information regarding energy consumption, and the information is deemed to be adequate to inform the prospective buyer, lessee, or lender regarding the United States environmental protection agency’s energy star portfolio manager benchmarking data and ratings for the most recent twelve-month period for the building that is being sold, leased, financed, or refinanced.”
-see http://apps.leg.wa.gov/RCW/default.aspx?cite=19.27A.170 

Start at the beginning: Chapter 19.27A RCW – Energy-related building standards

Also of interest:

RCW 19.27A.130 (put into effect in 2009) states the following:
“The legislature finds that energy efficiency is the cheapest, quickest, and cleanest way to meet rising energy needs, confront climate change, and boost our economy. More than thirty percent of Washington’s greenhouse gas emissions come from energy use in buildings. Making homes, businesses, and public institutions more energy efficient will save money, create good local jobs, enhance energy security, reduce pollution that causes global warming, and speed economic recovery while reducing the need to invest in costly new generation. Washington can spur its economy and assert its regional and national clean energy leadership by putting efficiency first. Washington can accomplish this by: Promoting super efficient, low-energy use building codes; requiring disclosure of buildings’ energy use to prospective buyers; making public buildings models of energy efficiency; financing energy saving upgrades to existing buildings; and reducing utility bills for low-income households.”  [2009 c 423 § 1.]

“By December 31, 2009, to the extent that funding is appropriated specifically for the purposes of this section, the department of commerce shall develop and recommend to the legislature a methodology to determine an energy performance score for residential buildings and an implementation strategy to use such information to improve the energy efficiency of the state’s existing housing supply. In developing its strategy, the department of commerce shall seek input from providers of residential energy audits, utilities, building contractors, mixed use developers, the residential real estate industry, and real estate listing and form providers.”    [2010 c 271 § 307; 2009 c 423 § 7.]